This invention relates to a butterfly valve which controls flow regulation of various fluids, and more particularly relates to the butterfly valve in which the valve body is constructed by synthetic resin material.
In the prior art, butterfly valves have been well known and used in handling various fluids, and generally they consist of a metal disk valve pivoted rotatably on a shaft in a cylindrical flow passage formed inside of a valve body, and between the valve body and the valve, and an elastic sealing material, a so-called seated ring, are adapted to open or close the flow passage and are adapted to control the fluid flow by contacting or detaching the outer circumferential surface of the disk valve on or from the inner circumferential surface of the seated ring.
The valve body equipped with such a butterfly valve having a cylindrical flow passage is generally provided as a one piece metal body by casting production or forging. The disadvantage to this one piece metal body is that it is quite difficult to obtain a light weight and compact body by e.g., reducing the inside thickness of the metal body during production. With regard to the casting production process, spruce burs or other burs are unavoidable during the process, which requires extra treatment inviting high costs in order to produce a smooth surface in the inner body or a smooth flow hole. Further, this casting process creates various problems such as unhealthy conditions or a bad environment for workers caused by harmful dust or residue.
In view of these situations, the present inventor has developed a valve body constructed of synthetic resin material, and including plural parts sectioning the body, and to join them together as a one piece valve body.
In an effort to realize light weight for these valve bodies, the following approaches were used in the past.
One prior approach teaches that the valve body and the valve be made of a compound material such as polyvinyl chloride or polypropylene or some other plastic material, but the valve body itself be produced as a one piece body, which does not differ at all essentially with the thought of producing a one piece metal valve body in the initial art. By this approach, in order to attain the mechanical strength for the valve body, it is necessary to form a projecting portion in the form of a rib-like member, and therefore, the total size of the valve body becomes too big in comparison with the diameter of the fluid passage.
There are also some other prior art approaches including a valve body made from synthetic resin material such as vinyl chloride and so on, but they are almost all produced in the similar method as explained above as a one piece body, and some are equipped with a similar rib-like member.
All of these prior art approaches provide a one piece valve body made from synthetic resin material, and the production method of a one piece body cannot be practiced in such a way as to eliminate the thickness of the inner parts. During the production procedure of synthetic resin products, when thickness is increased, it tends to cause bubbles therein, wherefore the cooling process invites the formation of hollowed portions. This serious problem is unavoidable during the production of a synthetic resin valve body produced by the prior art. In order to avoid this problem, the rib-like projecting portion is absolutely necessary to decrease the thickness. This projecting portion, however, is forced to encroach upon some larger space and thus the valve body becomes very big in comparison with the fluid passage, namely the effective diameter. This is really a fatal defect.
U.S. Pat. No. 4,826,133 discloses a butterfly valve body which is constituted by two parts, each part divided along the vertical line crossing the fluid passage and adapted to be completed by bolt-nut connecting each part. This patent is applicable to a metal valve body, but it is quite difficult or almost impossible to apply the same teaching to a valve body made from synthetic resin material. resin material suffers from shrinkage and expansion caused by water or hysteresis. According to the Patent, when two parts are combined, a kind of seated ring consisting of elastic sealing material is supplied therebetween, and the two parts are tightly locked by the bolt-nut connection, and therefore the sealing is attained by elastic deformation of the seated ring. Accordingly, if resin material were used instead of metal, the resin material has the nature of far larger shrinkage and expansion compared with metal, whereas the effective sealing would deteriorate with time, which tends to invite leakage of fluid badly during operation. Thus, the resin material is actually impossible to use according to the teaching of the patent.
In addition, the valve body is connected with a seated ring made from elastic sealing material and it is also fastened tightly by the bolt-nut means established at many points around the valve body, and thus, the valve body and the seated ring are assembled into one piece. Under this structure, when the replacement of the seated ring is necessary, the valve must be dismantled. In consequence, the exchange of the seated ring means the re-assembling of the whole valve body including the valve itself, which is quite troublesome and time consuming, such as unscrewing the bolt-nut means, refastening, fitting, adjusting, confirming and so on.
On the other hand, there lies a suggestion to use the very high quality resin which is called "Engineering Plastics", so that it guarantees the desirable sturdiness of the valve body without establishing the projecting portion with rib-like substance, but this kind of material is very expensive and is unsuitable for the expected cost of the butterfly valve. Moreover, even if this high quality resin is adopted, it still cannot eliminate other defects, such as the occurence of bubbles causing hollowed portions and so on.
In view of these serious problems, this invention has been developed.